Flask SECRET_KEY auto‑generation causing session loss after restart

Goal: Ensure that Flask user sessions remain valid across application restarts when moving from a local development setup to a production environment.

Constraint: If the SECRET_KEY configuration is omitted, Flask generates a new random key on each process start, which invalidates existing session cookies; local tests may not reveal the problem when server‑side sessions are unused, but production relies on persistent sessions for login state, CSRF protection and flash messages.

Uncertainty: While the documentation advises setting a constant secret, teams question whether a single static value shared across all instances is sufficient, or whether additional safeguards such as secure secret storage, key‑rotation procedures, or runtime change detection are needed to balance security with session consistency.

What is the recommended way to provision and distribute a stable SECRET_KEY across multiple production instances without exposing it in source control? How can operators verify that the configured key remains unchanged after restarts and scaling events? Are there any Flask-specific mechanisms to detect or warn when the secret key changes unexpectedly at runtime?